Super late reply, but I agree with all of this.
Regarding Star in particular, I think Mina being the last villain came out of left field and ultimately ended up as a serious missed opportunity. She was nowhere near as good of a villain as Toffee, mind you, or even on the level of Meteora or Ludo, but she had potential. They did a decent job setting her up as the final boss of the show before everything fell apart. Speaking of Ludo, they did an amazing job building up his character and threat level and telling his story in season 2. If they turned a villain who was once as pathetic, one-dimensional and harmless as Ludo into a such an amazing character, why couldn't they have done something similar for Mina? She fell flat as a villain in so many ways, but I wouldn't have minded her being the last villain if they better utilized her and any actual good writing was present in the last season. Moon, though...yeah, there was no reason for her to turn into an asshole. That came out freaking nowhere and was the most out of character thing the writers had ever done to one of the characters. It made no sense. There was no foreshadowing for it. And hell, her initial character arc in the last season alone pointed against it happening. There was no grounds for the twist. It was stupid.
Ah, and I concur. The Owl House and Amphibia are both really promising shows so far.